CHAPTER 14 The Origin of Species Modules 14.1 – 14.2

Evolution Underground Evolution has generally been thought of as a very gradual process However, examples of rapid evolution have been observed

One example of rapid evolution occurred among mosquitoes who migrated into the London underground In less than 150 years, Culex pipiens evolved into a new mosquito species, Culex molestus The origin of new species is called speciation

The isolated mosquitoes adapted to their new underground environment They altered their prey, mating habits (bred year round vs. winter hibernating above ground species), and breeding patterns When two different mosquitoes brought together in lab, no offspring = different species. Environmental barriers that isolate populations are just one of many mechanisms in the evolution of species

CONCEPTS OF SPECIES 14.1 What is a species? Linnaeus used physical appearance to identify species when he developed the binomial system of naming organisms (Genus species = Homo sapiens) This system established the basis for taxonomy, the naming and classifying of species.

But appearance alone does not always define a species Example: eastern and western meadowlarks They appear pretty much identical Figure 14.1A

Similarities between some species and variation within a species can make defining species difficult Humans exhibit extreme physical diversity, but we interbreed Mosquitoes no extreme physical diversity, but no interbreeding Figure 14.1B

The biological species concept defines a species as a population or group of populations whose members can interbreed and produce fertile offspring i.e. humans, although exhibiting considerable diversity of form

A ring species may illustrate the process of speciation 1 OREGON POPULATION Sierra Nevada 2 Yellow- blotched Yellow- eyed INLAND POPULATIONS COASTAL POPULATIONS Gap in ring Large- blotched Monterey 3 Figure 14.1C

Ring species Distribution forms a ring as it extends its range around a geographic barrier. http://www.youtube.com/watch?v=PjcFSy1KCTI

The biological species concept is not applicable to fossils or asexual organisms. So similarity of appearance, bio chemical features and fossil record are used to distinguish species. Most organisms are classified based on observable phenotypes The morphological species concept The genealogical species concept defines a species as a cluster of organisms representing a specific evolutionary lineage Uses DNA and RNA sequence data The ecological species concept defines a species by its ecological role

14.2 Reproductive barriers keep species separate Prezygotic and postzygotic reproductive barriers prevent individuals of different species from interbreeding Table 14.2

Courtship ritual in blue-footed boobies is an example of one kind of prezygotic barrier, behavioral isolation Many plant species have flower structures that are adapted to specific pollinators This is an example of mechanical isolation, another prezygotic barrier Figure 14.2A, B

Hybrid sterility is one type of postzygotic barrier A horse and a donkey may produce a hybrid offspring, a mule Mules are sterile Figure 14.2C

14.3 Geographic isolation can lead to speciation MECHANISMS OF SPECIATION 14.3 Geographic isolation can lead to speciation When a population is cut off from its parent stock, species evolution may occur An isolated population may become genetically unique as its gene pool is changed by natural selection, genetic drift, or mutation This is called allopatric speciation Figure 14.3

Changes occur by microevolutionary processes Mutation, genetic drift and natural selection Geographic isolation Mountain formation, deep canyons, removal of land bridges or continental drift Barrier effectiveness depends on effective dispersal in organisms that might speciate and size of the population Small populations more likely than large populations to develop into new species Large animals can cross some barriers a small animal might not

14.4 Islands are living laboratories of speciation On the Galápagos Islands, repeated isolation and adaptation have resulted in adaptive radiation of 14 species of Darwin’s finches Figure 14.4A

Adaptive radiation on an island chain 1 Species A from mainland 2 B A B 3 B B 4 C C C C D C D 5 Figure 14.4B

Darwin finches – distinguished by morphology and habitat Islands need to be close together or to mainland to allow for occasional dispersion but maintain mostly isolation Darwin finches – distinguished by morphology and habitat Cactus eaters Seed eaters Insect eaters Farther island with only 1 species, compared to islands close together with multiple

Unreduced diploid gametes 14.5 New species can also arise within the same geographic area as the parent species In sympatric speciation, a new species may arise without geographic isolation A failure in meiosis can produce diploid gametes Self-fertilization can then produce a tetraploid zygote Parent species Zygote Meiotic error Self- fertilization Offspring may be viable and self-fertile 2n = 6 Diploid 4n = 12 Tetraploid Unreduced diploid gametes Figure 14.5A

Sympatric speciation by polyploidy was first discovered by Dutch botanist Hugo de Vries in the early 1900s Figure 14.5B

Polyploidy Extra set of chromosomes Plants from tetraploid zygote can reproduce by self-fertilizaton but cannot produce fertile offspring by mating with diploid ancestors, due to resulting triploid offspring Most polyploid species are result of hybridization of two species with the resulting offspring being sterile. However, the hybrid may reproduce asexually.

14.6 Connection: Polyploid plants clothe and feed us Many plants are polyploid, including oats, potatoes, bananas, peanuts, barley, plums, apples, wheat and cotton They are the products of hybridization The modern bread wheat is an example Figure 14.6A

The evolution of wheat AA BB Wild Triticum (14 chromo- somes) Triticum monococcum (14 chromosomes) AB Sterile hybrid (14 chromosomes) Meiotic error and self-fertilization AABB DD T. turgidum EMMER WHEAT (28 chromosomes) T. tauschii (wild) (14 chromosomes) ABD Sterile hybrid Meiotic error and self-fertilization AA BB DD T. aestivum BREAD WHEAT (42 chromosomes) Figure 14.6B

14.7 Reproductive barriers may evolve as populations diverge This has been documented by laboratory studies (fruit flies) Flies more likely to breed with flies raised on same medium Demonstrates development of reproductive barrier w/in two populations of flies Initial sample of fruit flies Starch medium Maltose medium Results of mating experiments Female populations Female Starch Maltose Same Different Starch Same 22 9 18 15 Male populations Male Maltose 8 20 12 15 Different Mating frequencies in experimental group Mating frequencies in control group Figure 14.7A

examples in natural populations (pupfish in Death Valley) Each spring has unique species Figure 14.7B

14.8 The tempo of speciation can appear steady or jumpy According to the gradualist model of the origin of species new species evolve by the gradual accumulation of changes brought about by natural selection However, few gradual transitions are found in the fossil record Figure 14.8A

The punctuated equilibrium model suggests that speciation occurs in spurts Rapid change occurs when an isolated population diverges from the ancestral stock Virtually no change occurs for the rest of the species’ existence Figure 14.8B

Ultimately both models have some merit, and some species evolved with a combination of these models.

14.9 Talking About Science: Peter and Rosemary Grant study the evolution of Darwin’s finches The occasional hybridization of finch species adds to the genetic variation of parent populations This may have been important in the adaptive radiation of finch species Figure 14.9